The desirable goal of increasing the areal recording density of magnetic disk drives frequently can be obtained only with an undesirable side effect, i.e., increased raw error rate. For example, increased areal recording density can frequently be obtained by reducing the distance between the recording head and the media surface during periods of data transfer. This distance is known in the art as the "flying height." Reducing the flying height increases the susceptibility to errors due to media surface defects or debris that can cause the head to momentarily alter its flight profile. If this occurs during a write operation--known as a "high-fly write"--the resulting data written on the media can, during a subsequent read operation, exhibit errors exceeding the correction capability of the drive electronics or can be written with such marginality that the reliability of subsequent reads of the data is unpredictable. These transient errors can significantly impact the performance of the disk drive.
Historically, the design of magnetic disk drives has focused on improvements in read signal processing to optimize the capture and correction of data recovered from the media, without evaluation of the data originally written. There are limits to the ability of read electronics to recover marginal data, including the length of error correction codewords on the media which subtracts from the area available for user data; the dynamic range of channel electronics; and the tolerance of decoding schemes such as PRML to erroneous data. These limits are challenged by the inexorable march towards higher areal density and increasing demands for reliability in the face of severe price competition.
Although disk drives provide for post processing or "heroic recovery" of data stored in the disk drive buffer by the drive microprocessor, performance is degraded in these instances due to computing cycles which must be executed by the microprocessor and the need to occupy the limited buffer space available in the drive.
In other types of storage peripherals the verification of data written to the media, at the storage device level, has been limited to magnetic tape systems and optical disk drives. Magnetic tape practitioners have dealt with the problem of imperfect writes with a head/media structure having separate read and write heads with the read head positioned downstream of the write head. This allows the data written on the media to be verified simultaneously with the write operation.
Optical disk drives, being used primarily for archival or read-only applications, have provided data integrity checks that compare buffered host data with data read from the media. This was done because the writing process was known to be error prone and not a factor in performance, since only the time required for read operations is perceived by the user in most instances. For example in U.S. Pat. No. 5,471,351, Ishiguro discloses a method for verifying data being written by writing the data, then comparing data read from the media to the buffered data to be written, counting errors, then comparing those errors to the number of errors recorded from an ECC error correction on the same data. This method imposes a heavy processing load on the disk microprocessor as well as requiring additional storage space for the data read from the media.
In U.S. Pat. No. 5,469,418, Satoh et al disclose a dual head system for verification of data read from the media, a costly structure not compatible with the cost constraints of a competitive magnetic disk drive. A comparable approach, suitable only to optical drives, is disclosed in U.S. Pat. No. 5,406,540 by Longman et al, who use a photodiode as a read element.
In U.S. Pat. No. 5,184,341, Hamasaka et al disclose a method for successively writing and verifying data in an optical disk drive that performs verification by reading and processing previously written data including the detection of uncorrectable errors. However Hamaska et al teach that the recorded data to be verified is stored in a buffer memory and do not suggest the use of on-the-fly error correction techniques.
The existing art in the field of storage peripherals therefore does not suggest how to satisfy the requirements of verifying written data within a magnetic disk drive without significant use of the limited buffer space provided for data storage and while minimizing the processing load on the disk drive microprocessor.